The present invention relates to a method for transmitting a tributary in synchronous mode at the level of a link of a data network.
Tributaries correspond to a data flow received or emitted by some equipment at the level of a link in a data network. The transmission of a tributary in synchronous mode may concern a time division multiplexing which consists in grouping several asynchronous tributaries having a first rate on a single container having a higher rate so as to make common a transmission carrier. The transmission of a tributary in synchronous mode may also relate to a time division inverse multiplexing which consists in fragmenting a tributary onto several channels with lower rates which will be subsequently realigned to reconstitute the original tributary. This technique makes it possible to facilitate the transport of high rate signals on channels where the propagation frequency is limiting. The transmission of a tributary in synchronous mode also relates to the regeneration of a tributary which consists in transmitting a tributary at the same rate.
Synchronous flows are either flows of the SDH “Synchronous Digital Hierarchy” type or flows of the SONET “Synchronous Optical Network” type. Whereas the SONET concept was developed in the United States, the SDH hierarchy is, in particular, the subject of UIT-T (Union Internationale des Télécommunications=International Telecommunication Union) recommendation G.707. The following part of the present description will deal with the SDH hierarchy but as the SONET and SDH concepts are close to each other, it is within the range of the persons skilled in the art to extrapolate what will be said about the SDH hierarchy to the SONET concept. The advantage of the synchronous transfer mode, as it is indicated in the SDH hierarchy and in the SONET concept, lies in the structure of the used frames which is similar throughout the hierarchy and which allows the aggregation of different flows by interlacing after an adaptation of the rate using a mobile pointer making it possible to identify the beginning of the payload in each transport container. For further details about the SDH hierarchy or the SONET concept, please also refer to the above mentioned recommendation G707.
The SDH or SONET standards more particularly teach a method for transmitting at least one tributary in synchronous mode at the level of a link of a data network, said tributary comprising at least one frame in the SDH or SONET format, said frame comprising at least one payload envelope area and one transport overhead area, said payload envelope area comprising at least one payload and load supervision data related to the payload, said transport overhead area comprising at least transport supervision data related to transport and an area and fixed patterns (A1, A2) allowing the alignment of said frame in reception, said method including steps of:    modifying said frame so as to generate a modified frame;    transmitting said modified frame.
During a link in the form of a multiplexing using a multiplexer in the SDH or SONET standard, the supervision data comprised in the field of the transport overhead of each tributary are processed then deleted and then replaced by new supervision data corresponding to a higher level signal in the hierarchy. This requires an adjustment of the rate of the tributary trains by modifying the pointer and the position of the beginning of the payload in the frame. During the multiplexing operation, the modification of the pointers of each of the tributaries has an impact on the contents of the train and the position of the payload in the transport container, so as to adjust the rate of the tributaries to the local rate of the multiplexer.
Thus, the multiplexing method previously described has the disadvantage that the tributaries are not really transmitted with a fixed rate and contents. This more particularly prevents a transparent transmission of the tributaries in the network and affects the needed integrity of the signal transmitted mentioned by some users. The aim of the invention is more particularly to remedy this drawback.
To keep a content tributary strictly identical during the transmission, the tributary itself could be considered as a payload and an overhead could be applied thereto, thus enabling the insertion of supervision data. However, the overhead would make the obtained frame non compatible with the SDH or SONET standards and would thus make it impossible to use the invention on existing networks.
Thus, the aim of the invention is to allow a transparent transmission of a tributary in synchronous mode at the level of a link of the data network. Another aim of the invention is also to provide equal frequencies at the input and the output of a link of a data network. Another aim of the invention is to allow a transparent transmission of a tributary in synchronous mode at the level of a link of a data network while keeping compatibility with the SDH or SONET standards.
The document EP 1246383 A1 is known in the state of the art. This document describes a system of transmission by data multiplexing in the form of SDH or SONET frames. For the purpose of the multiplexing and the adaptation of the transmission rate after multiplexing, a sequence of fixed patterns of frame is replaced by another sequence of adaptation having a variable size, with the sequence including controlled and management data. However, in the above-mentioned document, the coherence of the tributary flow in reception after the inverse multiplexing is not kept. The invention also aims at remedying this drawback.
Therefore, the invention relates to method of transmitting at least one tributary in synchronous mode at the level of the link of the data network, said tributary comprising at least one frame in the SDH or SONET format, said frame comprising at least one payload envelope area and one transport overhead area, said payload envelope area comprising at least one payload and load supervision data relating to the payload, said transport overhead area comprising at least transport supervision data related to transport and an area of fixed patterns comprising fixed patterns, said method comprising steps of:    modifying said frame so as to generate a modified frame;    transmitting said modified frame, the modification comprising the steps in which:    part of said fixed patterns are deleted from said fixed pattern area;    the positions of said part on the fixed patterns deleted are recorded;    replacement data are inserted at the level of said positions,wherein said replacement data includes transport supervision data of the frame of said tributary.
Thus, by deleting a part of the fixed patterns, space is cleared to insert replacement data. This replacement then makes it possible to keep fixed the rate at the level of the link. In addition, in the case of the inverse multiplexing, thanks to the fact that the replacement data include the transport supervision data of the frame of said tributary, the coherence of the tributary flow in reception after an inverse multiplexing is kept. On the contrary, the document EP 1246383 A1 mentioned above does not relate to the inverse multiplexing and does not make it possible to keep the coherence of the tributary flow in reception after an inverse multiplexing.
The deleted part of the fixed patterns exactly corresponds to a quantity required for the insertion of the replacement data, so that the rate of tributaries is strictly fixed when going through the link. The replacement data are then adapted as a function of the type of the transmission of the tributary. The invention thus processes advantageously the fixed patterns of an SDH or SONET frame by adding replacement data to a fixed pattern field. The invention can be applied to any type of transmission at the level of a link and more particularly to a time division multiplexing, a time division inverse multiplexing or a signal regeneration.
According to one embodiment of the invention, in said method said replacement data include said supervision data and said method includes steps in which:    said supervision data are stored;    after the storing step, said supervision data are replaced by new supervision data.In this case, the replacement data correspond to the supervision data themselves. This makes it possible to remain compatible with the SDH standard in spite of the introduction of new supervision data. This embodiment is advantageously implemented in the case of an inverse multiplexing.
The deletion of fixed patterns in the area of fixed patterns also has the advantage of clearing some space for proprietary data related for example to services while complying with the rate constraints. For the purpose of the present invention, external data are thus defined as data not included in the process frame. Such external data are distinguished for example from supervision data which are included in the frame. In this case, in the method of the invention, said replacement data include external data.
In order to be adapted to the SDH format in the above mentioned method, said tributary comprising at least one frame in the SDH format and said fixed patterns are alignment words. According to this embodiment, alignment words are deleted. The replacement data thus replace these deleted alignment words.
In addition, said transmission includes an emission and a reception wherein upon the reception, fixed words are reinserted at the level of said positions. This makes it possible then upon the reception to obtain a tributary exactly similar to the transmitted tributary. The coherence is thus guaranteed according to the method of the invention. In this case, upon the reception, it is possible to reintroduce said replacement data into the transport supervision data area.
According to one embodiment, the method includes a step of introducing a delay, said replacement data being inserted after said introduced delay. The invention also relates to a device for the transmission of at least one tributary in synchronous mode at the level of a link of a data network, said tributary comprising at least one frame in the SDH or SONET format, said frame comprising at least one payload envelope area and one transport overhead area, said payload envelope area comprising at least one payload and load supervision data linked to the payload, said transport overhead area comprising at least transport supervision data related to the transport and an area of fixed patterns comprising fixed patterns, said method including:    means for modifying said frame so as to generate a modified frame;    means for transmitting said modified frame,    means for deleting a part of said fixed patterns in said fixed patterns area;    means for recording the position of said part of said deleted fixed patterns;    means for inserting replacement data at the level of said positions,wherein said modification means are so arranged that said replacement data include said transport supervision data of the frame of said tributary.
According to a second aspect, the invention relates to a point to point transmission of data, wherein it is not necessary to guarantee the compatibility with SDH or SONET technology transport elements. Thus, according to this aspect, the tributaries are not necessarily compatible with these formats. As a matter of fact, in this case, maintaining the SDH frequency makes it possible in itself to use current SDH equipment which are not expensive.
Thus, according to this second aspect, the invention relates to a method for transmitting at least one tributary in synchronous mode at the level of a link of a data network, said tributary including at least one frame, one frame including at least one payload envelope area and one transport overhead area, said payload envelope area comprising at least a payload and load supervision data related to the payload, said transport overhead area comprising at least transport supervision data related to the transport and an area of fixed patterns including fixed patterns, said method comprising steps of:    modifying said frame so as to generate a modified frame;    transmitting said modified frame,the modification comprising at least one step in which:    a part of said fixed patterns are deleted from said fixed patterns area, so as to reduce the rate of the tributary.
Thus, according to this aspect of the invention, the fixed patterns are used themselves as information for the adjustment of the rate when passing through a transmission device more particularly for the multiplexing or regeneration. As a matter of fact, according to this aspect of the invention, the deletion of a part of the fixed patterns makes it possible to adapt the rate of the tributary to the transmission device during the multiplexing or the regeneration. According to this aspect of the invention, the fixed patterns are deleted without being replaced by other data, so that the result is a reduction in the rate.
On the contrary, in the document EP 1246383 A1 mentioned above, in the case of a multiplexing, fixed patterns are deleted, but they are replaced by patterns having the same length at locations temporarily used for other purposes. The result is thus a constant rate which does not make it possible to adapt to the rate of the multiplex transmission device. In addition, in the above mentioned document, the compatibility with the SDH format is an essential aspect, whereas the pure deletion of the invention without any replacement does not make it possible to maintain this compatibility.